As described, there is a restriction in the hot water line going only to the shower. This wasn't a complete blockage and some flow existed. When the shower valve was shut off, the slowed flow had time to build up pressure resulting in the "jump" described as the pressure build up was released and not supported by enough flow.
If the cartridge was removed as indicated, somehow the water had been shut off. Based on the description of water coming out of the hot side (no indication of how much) it would indicate the shut-off valves leading to the shower were used. If in fact the hot water flow indicated was restricted with the cartridge removed, this would indicate the restriction was at the shut-off valve or in the pipe leading to the shower valve assembly. This would indicate either the shut-off valve was defective or something was restricting the flow to the shut-off valve or shower valve assembly.
If this occurred after years of trouble-free operation, the most likely problem would be the failure of the shut-off valve (mineral deposit build up would show at other hot water outlets as well and therefore not likely the problem). Repair or replace the valve.
If this occurred right away or a short time after installation, it might be a defective shut-off valve or what I call sloppy plumbing and the plumber got something in the pipe during installation. I have encountered this when repairing a similar problem. It turned out that the plumber wasn’t careful to cover an open pipe when drilling a hole to run another pipe. The result was chunks of wood that finally plugged the shower valve. Another possibility is that excess solder from the sweating process somehow got stuck in the valve opening. Depending on the type of valves, it might be possible to open them up and use compressed air to clear the lines. Most likely the pipes/valves would need to be removed and cleaned or replaced. The process for doing this should be from the shower first and going toward the water source, checking the flow after each part of the line is removed if the blockage is found.